At low angles-of-attack the steering and roll characteristics of a missile are essentially independent. At such angles, rotation of the missile about its longitudinal axis, i.e., roll, has no significant effect on the direction of flight, or steering, of the missile and, conversely, steering effects have negligible effect on missile roll.
At higher angles-of-attack, however, an unintentional undesirable cross-coupling between steering and roll exists. Several aerodynamic flow phenomena are responsible for this undesirable effect. First, vortices are created by air streaming back from the body of the missile over the wing and tail surfaces; vortices are also shed from wing surfaces and stream back over the tail surfaces. These vortices produce large unwanted torques which increase with the angle-of-attack. Second, during large direction changes, the windward side of the missile experiences a great increase in pressure not exerted on the leeward side. The lower wing and tail surfaces may then be in a pressure field of greater intensity than the field surrounding the upper surfaces. Each of these results causes instability which must be compensated for or otherwise eliminated.
One technique employed is to interdigitate wing and tail surfaces offsetting them by, for example, 45.degree., the wings being in an + and the tail fins being in a X orientation. This method, while addressing the problem of vortices, does not specifically compensate for pressure differentials between windward and leeward surfaces and is ineffectual where airframe structure is otherwise defined or limited by overall system design constraints.
Another technique, described in U.S. Pat. No. 3,946,968, considers the effects of aerodynamic coupling due to unequal pressure, vortices, and downwash on control fins. Unlike the present apparatus, U.S. Pat. No. 3,946,968 ignores the effects of undesired coupling due to fixed surfaces, such as strakes and dorsal fins. The technique of the reference is to determine the pressure on control surfaces with strain gauge measurements and to provide force and moment feedback inputs to equalize pressure on the inplane control fins. The reference provides force commands to servomechanisms which reposition the fins instead of employing fin angle or rate commands as in the invention. A difficulty associated with this method is in designing a transducer that responds as required, as pointed out in April 1961, by L. L. Cronvich and B. E. Amsler in an article "Pitch-Yaw-Roll Coupling" (Report 353 (AD 448911) of the Advisory Group for Aeronautical Research and Development, NATO) which discussed the problems relating to the technique of equalizing the forces developed by inplate fins with transducer means. U.S. Pat. No. 3,949,968 discusses the cross-feeding of roll, yaw, and pitch servo outputs back into the servo inputs. The cross-coupling of or cross-feeding of the outputs of the pitch, roll, and yaw-sensing instruments is, however, not contemplated or suggested. Reliance on the servo feedback along limits effective decoupling and fails to satisfy required stability and performance needs. Thus, although mindful of stability problems at high angles-of-attack, the referenced patent, in devising a control system based solely on force parameters and providing for cross-feeding only servo signals, fails to satisfy high angle problems in general and provides no satisfactory solution to high attack angle problems in existing cruciform configured missiles in specific.
The prior art includes still another means for confronting high attack angle difficulties. By increasing roll bandwidth relative to the steering bandwidth, decoupling between the roll and steering systems can be increased. However, increased bandwidths result in increased noise levels. Further, elevations gain to raise the signal strength is required. Gain changing is effective within limits. Beyond those limits component saturation occurs and the system no longer operates satisfactorily. The limits are exceeded at high angles-of-attack. Similarly, decreasing steering bandwidth increases response time beyond limits amenable to effective target engagements.
Finally, various references, such as U.S. Pat. No. 3,951,358, include error signals, oscillation damping, and missile rolling within limits. Instead of providing roll stabilization, such systems allow the missile to roll at a predetermined rate set by a gyro unit. The premise of these known techniques is that complete roll stabilization is not necessary. Although this premise may be a reasonable one at low and medium angles-of-attack, the cross-coupling effects and instability introduced by even limited roll generate adverse effects at high angles.
Despite the various approaches of changing fin configuration, cross-feeding servo responses, increasing bandwidth, measuring pressure differentials, limiting roll rate, damping oscillations, and the like, no adequate method or apparatus for controlling, without loss of stability or response, a missile attacking at a high angle has been disclosed in the prior art.